This invention relates to a method and apparatus for providing a quantitative measure of the existence of a crevice or gap between an inner metallic tube and an outer, thick sheet of metal called a tubesheet and, more particularly, to the use of the principles of electricity based on Ohm's Law governing the passage of current through materials to obtain such quantitive measurements.
In the design of steam generators, the surface area for exchanging heat is maximized through the use of metal tubing as a barrier between the primary and secondary fluids used for heat exchange. The tubing can be assembled into a configuration and coupled to the primary fluid or coolant, so that the primary coolant can pass therethrough. The second fluid or coolant can be arranged about the outside of the tubing. The steam generator can be constructed of many lengths of tubing assembled in a bundle which connects to a source of heat. In such an assembly of tubing bundles, the ends of the tubing can pass through a thick sheet of metal, which can be called a tubesheet, which retains the tubing in position and forms a barrier between the primary and secondary fluids.
Generally, a heat exchanger is constructed by installing and fastening each tube into the tubesheet. For example, the ends of the tube can be inserted into pre-drilled holes in the tubesheet. The tubes can be expanded or "rolled" from the inside of the tube until the outside walls of the tubing make intimate contact with the walls of the tubesheet hole. The space between the tube and tubesheet must be closed to prevent the formation of a crevice between the tube and tubesheet which would promote corrosion.
In such a heat exchanger, it is important to assure that there is no gap or crevice, namely no space, between the tube and the tubesheet. An available technique for determining if the crevice or gap is closed (i.e.. there is no space between the tube and tubesheet). is to measure the internal diameter of the tubing using Eddy Current Techniques. The gap is then inferred from the difference between the nominal size of the tubesheet hole and the measured internal diameter of the tube assuming the exact thickness of the tube wall is known. Thus: EQU Crevice=Hole Diameter-(Tube Internal Diameter+Wall Thickness)
The calculation must assume that all the tubesheet holes are of the same diameter and all of the tubing walls are of the same thickness. However, the thickness of the tubes and the inner diameter of the tubesheet holes are not constant because these values are subject to manufacturing variations which allow differences of several thousands of an inch. Accordingly, this technique can not assure that there is no gap between the tube and tubesheet hole. especially if there is a failure to fully expand the tube. For example, steam generators have been manufactured which have expansion differences of 0.003 to 0.016 inch.